First decide what voltage you are powering it off. This will be the maximum voltage on the output, you need to cut this down to 5V maximum.

Power it up with +ve and -ve and make sure the -ve is connected to the Arduino ground.

Then arrange a potential divider to take the output and reduce it to a maximum of 5V. If it's 10 volts then simply two 1K resistors will do.

Then connect the mid point in the potential divider to an analogue input pin of the Arduino and read the value. In the presence of metal it should be quite low (close to zero) and with no metal it should be high. (above 700)